1. Field of the Invention
The present invention is directed generally to micro-electromechanical system devices (MEMS devices), and more particularly to MEMS devices having an integral electrical isolation structure.
2. Background Art
MEMS devices are electrical and mechanical devices that are fabricated at substantially microscopic dimensions utilizing techniques well known in the manufacture of integrated circuits. Present commercial applications of MEMS devices are predominantly for pressure and inertial sensing, for example, accelerometers and gyroscopes used in hand-held devices, for example, cell phones and video game controllers.
For example, a MEMS device that is an accelerometer can detect when the cell phone experiences acceleration such as when the phone is rotated from a portrait orientation to a landscape orientation. Such a inertial sensing MEMS device can include a case or substrate, a mass resiliently held within the case, and a deflection sensor for measuring relative motion between the case and the mass. When an acceleration is experienced, the mass moves relative to the case, and the sensor measures the deflection. In most cases, the acceleration is directly proportional to the amplitude of the deflection. Processing steps have been developed to make a MEMS device having such a mass and deflection sensor. When a MEMS device is constructed using processes such as the one disclosed in U.S. Pat. No. 6,239,473 to Adams et al., silicon beams coated with silicon dioxide on three sides can be formed. These beams can have an isolation joint that moves with the rest of the structure. These isolation joints enable multiple electrical signals to be routed to multiple places within a device and applied to multiple electrical components such as sensors and actuators. However, MEMS devices fabricated according to the process outlined in U.S. Pat. No. 6,239,473 are susceptible to shock damage, interconnect damage, and frit seal failure.
Shock Damage
One cause of shock damage in an inertial sensing MEMS device relates to a dielectric coating on the sidewalls of the beams. If subjected to large accelerations, for example, when a cell phone or game controller strikes the ground after being dropped, the sidewalls of the beams can contact each other, causing the dielectric coating to wear by chipping or abrasion. During the wear process, chemical bonds between molecules in the dielectric sidewall coatings are broken, creating an electrical charge on the sidewall surface. Because these sidewall surfaces are often silicon dioxide, an insulating material, the electrical charges do not dissipate quickly. The charges can persist for hours or even days after the mechanical shock occurred. At the size scale of MEMS devices, these charges can affect the operation of the MEMS device.
An electrical charge on the outer surface of the dielectric sidewall coatings can causes a net force on the beam. This net force is indistinguishable from an acceleration that causes the beam to move. Therefore, a charged device produces a false acceleration signature.
Interconnect Damage
In addition to damage to the dielectric sidewall coatings, offset shifts can also be created by a permanent plastic deformation or bend in the metal used to electrically interconnect various portions of the MEMS device from the application of large forces during operation. Plastic deformation of the interconnect metal can causes the entire beam to deform, which can cause a perceived offset shift and a false acceleration signature.
The interconnect metal can also be deformed by large temperature excursions. MEMS devices fabricated using the process discussed in U.S. Pat. No. 6,239,473 comprise multiple materials, for example, silicon dioxide, silicon, and aluminum. Each of these materials has a different coefficient of thermal expansion, meaning that as the temperature changes, each material expands different amounts. Because the materials are joined together, the materials all deform approximately the same amount, causing a stress. If the stress levels are large enough, the materials can permanently deform. Aluminum deforms easier than either silicon or silicon dioxide. Accordingly, when a MEMS device is subjected to high temperatures excursions, for example, temperature excursions during the solder reflow cycles, the aluminum can plastically deform, causing a perceived offset shift and a false acceleration signature. The actual amount that a device deforms depends on the structural design and the quantity of metal used. For example, an accelerometer fabricated using the process discussed in U.S. Pat. No. 6,239,473 moves about 20 nm per g of acceleration. Due to the plastic deformation of the metal during reflow, the rest position of the accelerometer may shift up to 2 nm which is equivalent to a false reading of 100 mg's.
Minimizing the thickness of the interconnect metal can reduce the deleterious effects. However, in U.S. Pat. No. 6,239,473, the metal bond pads and the metal seal ring surface are formed from the same layer of metal comprising the interconnect metal, and the metal bond pads and the metal seal ring surface have minimum thickness requirements to function properly. Thus, a solution for reducing interconnect damage is not as simple as merely reducing the thickness of the metal layer forming the interconnect.
Seal Failures
MEMS devices such as those described in U.S. Pat. No. 6,239,473 use a lid to form a hermetic seal around the beams of the substrate. The lid can be coupled to the substrate using a frit seal that interfaces with a metal seal ring surface. If the interface between the frit seal and the metal seal ring surface is interrupted, for example, by a metal trace running directly under the metal seal ring surface to a bond pad, the interface between the lid and the substrate is weakened. The interface can also be weakened when the metal traces are covered locally with a passivation oxide to prevent any electrical interactions with the lid or fit seal. Accordingly, when a MEMS device having an interrupted interface between the lid's frit seal and the metal seal ring surface is subjected to excessive environmental stresses, the MEMS can fail caused by the weakened seal.
Accordingly, there is need for improved MEMS devices that can better withstand mechanical shocks, reduce the risk of metal interconnect damage, and provide improved frit seals.